Using the A1c Test to Diagnosis Diabetes

We do a fasting and it looks good, we do a 2 hour glucose test and it is fine but then the A1c comes back at 7%: does the patient have diabetes or not? Randie R. Little, Associate Professor University of Missouri, Director Diabetes Diagnostic Laboratory, says we could be Using the A1c Test to Diagnosis Diabetes. See if you agree with her logic

Using the A1c Test to Diagnosis Diabetes

Randie R. Little is an Associate Professor in the Departments of Pathology & Anatomical Sciences and Child Health at the University of Missouri and is Director of the Diabetes Diagnostic Laboratory. Dr. Little is the coordinator of the NGSP network and is a member of the NGSP Steering Committee and the IFCC working group on HbA1c.

Randie R. Little

The global prevalence of diabetes mellitus is increasing rapidly. In the US, 23.6 million people (7.8% of the population) have diabetes. Of those with diabetes in the US, it is estimated that 5.7 million people (25%) do not know that they have it! In addition, there is on average, a seven year lag between onset and diagnosis; consequently, 25% of newly-diagnosed patients already have diabetic complications such as retinopathy or microalbuminuria. Earlier diagnosis of diabetes could prevent or delay these complications. Many believe that diabetes is not being diagnosed effectively and that better tests for screening and diagnosis should be considered.

HbA1c is used to monitor long-term glycemic control, adjust therapy, and assess risk for the development and progression of diabetic complications. Most diabetes organizations worldwide recommend specific HbA1c treatment goals; the test is fundamental to the management of these patients. HbA1c is usually reported as a percentage of total hemoglobin. NGSP-standardized results, which are equivalent to those reported in the Diabetes Control and Complications Trial (DCCT) and the United Kingdom Prospective Diabetes Study (UKPDS), are used most widely. This enables a patient’s result to be directly related to these clinical outcome studies where the relationships between HbA1c and diabetes complications have been demonstrated.

Currently diabetes (in the absence of symptoms) is diagnosed using fasting plasma glucose (>126 mg/dl) or an oral glucose tolerance test (2-h glucose >200 mg/dl). However, plasma glucose demonstrates significant variation within individuals; it is for this reason that either of these tests must be confirmed by repeat testing on a different day. A review of HbA1c (also called the A1c Test) as a screening tool for detection of Type 2 diabetes concluded that HbA1c is as effective as fasting plasma glucose for the detection of Type 2 diabetes when using the oral glucose tolerance test as the gold standard (1). The fact that the test can be done without fasting is one of its biggest advantages; this will certainly increase the opportunity to diagnose diabetes. HbA1c also shows much lower variation within individuals compared to blood glucose. In the past, the fact that the assay was not standardized hindered its widespread use for diabetes screening or diagnosis. However, with the NGSP in place for the past 12 years (2), the variability of the test among laboratories has improved dramatically. An expert committee has recently recommended that HbA1c be incorporated into criteria for screening and diagnosis of diabetes. This committee suggested that HbA1c >6.5% would be diagnostic of diabetes if confirmed by an increased blood glucose value (3). The American Diabetes Association (ADA) and European Association for the Study of Diabetes (EASD) have established a joint committee to re-evaluate the diagnosis of diabetes. It is likely that they will soon recommend HbA1c as a screening/diagnostic test for diabetes; more details are expected to be released in the coming months.

For the vast majority of people with diabetes, HbA1c provides an excellent measure of glycemic control and it would also make an excellent test for screening/diagnosis of diabetes. However, there are situations where HbA1c may be unreliable and these must be considered with more widespread use of the test. These include any condition that alters the lifespan of the red blood cell (e.g., hemolytic anemia), severe iron-deficiency anemia, and certain hemoglobin variants (e.g. HbS or HbC) or adducts. Interference from hemoglobin variants is method-specific and reporting of inaccurate results can be avoided by careful use of appropriate testing methods. Further studies are needed to clarify the role of HbA1c in diabetic patients with chronic renal failure as well. Factors such as race or age have also been reported to influence HbA1c to a small extent.

The dramatic improvement in the quality of HbA1c testing over the past 10-15 years, coupled with the advantages HbA1c offers (fasting not required, low within-person variation) have now led us to the point where HbA1c is being acknowledged as a suitable tool for the screening/diagnosis of diabetes. This will hopefully facilitate wider implementation of diabetes screening that will, in turn, reduce the human and economic costs associated with the complications of diabetes.